If your hens have injured a new hen, I would recommend separating the new hen immediately and examine her for injuries. An injured or sick hen should always be separated, both to protect your flock from illness and to protect a weaker bird from being attacked.
Chickens are naturally territorial flock animals and will often have issues with new birds coming in; it's rather tricky to just throw a new one in with an existing flock. I personally would not introduce a single bird (alone) into a new flock, as it's generally a recipe for much unhappiness on the new bird's part, unless he/she is very dominant.
check her for injuries..feel for any swelling or heat..
make sure you check the bottoms of the feet for Bumblefoot..(sore or scab on pad of the foot)
separate her to a warm safe place with soft bedding..and make up this mix:
layer feed, water cooked oatmeal, plain yogurt, cooked egg yolk..
do you have any vitamins?
describe the droppings..color and consistency..
sometimes worms can cause them to have weak legs.
try and see which of your hens is the most docile to become a companion..maybe keep her next to the new hen for a few hours a day..
also a temporary pen with the others where she can be seen but not attacked might help transition..